Ultrasonic waves have been widely used in medical applications, including both diagnostics and therapy as well as many industrial applications. One diagnostic use of ultrasound waves includes using ultrasonic waves to detect underlying structures in an object or a human tissue. In this procedure, an ultrasonic transducer is placed in contact with the object or tissue via a coupling medium and high frequency (1-10 MHz) ultrasonic waves are directed into the tissue. Upon contact with various underlying structures, the waves are reflected back to a receiver adjacent the transducer. By comparison of the signals of the ultrasonic wave as sent with the reflected ultrasonic wave as received, an image of the underlying structure can be produced. This technique is particularly useful for identifying boundaries between components of tissue and can be used to detect irregular masses, tumors, and the like.
Two therapeutic medical uses of ultrasound waves include aerosol mist production and contact physiotherapy. Aerosol mist production makes use of a nebulizer or inhaler to produce an aerosol mist for creating a humid environment and delivering drugs to the lungs. Ultrasonic nebulizers operate by the passage of ultrasound waves of sufficient intensity through a liquid, the waves being directed at an air-liquid interface of the liquid at a point underneath or within the liquid. Liquid particles are ejected from the surface of the liquid into the surrounding air following the disintegration of capillary waves produced by the ultrasound energy. This technique can produce a very fine dense fog or mist. Aerosol mists produced by ultrasound are preferred over aerosol mists produced by other methods because a smaller particle size of aerosol can be obtained with the ultrasonic waves. One of the major shortcoming of inhalers and nebulizers is that the aerosol mist cannot be directed to a target area without an air stream, which decreases the efficiency of ultrasound. Ultrasonic sprayers such as those sold by Sonic and Materials Inc., Misonix Inc., Sono-Tek Inc. (see, for example, U.S. Pat. Nos. 4,153,201, 4,655,393, and 5,516,043) operate by passing liquid through a central orifice of an ultrasound instrument-tip. Major disadvantages of these sprayers include non-uniform particle size, heating of liquid flow, and less efficiency of ultrasound waves because of demolished end (radiation) surface configuration of the tip.
Contact physiotherapy applies ultrasonic waves directly to tissue in an attempt to produce a physical change in the tissue. In conventional ultrasound physiotherapy, an ultrasonic wave contacts the tissue via a coupling medium. Ultrasonic waves produced by the transducer travel through the coupling medium and into the tissue. The coupling medium is typically a bath of liquid, a jelly applied to the surface to be treated, or a water-filled balloon. Conventional techniques provide ultrasonic waves having an intensity of about 0.25 w/cm2 to 3 w/cm2 at a frequency of about 0.8 to 3 Megahertz. The treatment is applied to a skin surface for from about 1 to 30 minutes, two or three times a week. The coupling medium can provide a cooling effect which dissipates some of the energy produced by the ultrasonic transducer.
More importantly, a coupling medium or direct contact between the tissue and ultrasonic transducer is necessary to transmit the ultrasonic waves to the skin surface because ambient air is a relatively poor medium for the propagation of ultrasonic waves.
Several beneficial effects have been reported from contact ultrasound physiotherapy, such as, for example, the following: local improvement of the blood circulation, heating of the tissue, accelerated enzyme activity, muscle relaxation, pain reduction, and enhancement of natural healing processes. Despite these beneficial effects, current techniques of medical physiotherapy using ultrasonic waves are limited by the necessity of providing a direct contact interface between the ultrasonic transducer and the tissue to maintain an effective transmission of the ultrasonic waves from the transducer to the tissue.
The necessity of direct contact with or without a coupling medium makes current methods undesirable. Some tissue conditions may be accessible to contact ultrasound devices but would be impractical for contact ultrasound treatment. For example, fresh or open wounds resulting from trauma, burns or surgical interventions are not suitable for direct contact ultrasound treatment because of the structural nature of the open wound and the painful condition associated with those wounds. Moreover, conventional contact ultrasound may have a destructive effect on these types of open wounds due to the close proximity of an oscillating tip of an ultrasonic transducer relative to the already damaged tissue surface.